This invention relates generally to a method and apparatus for transferring blanks, plastic parisons, or other moldable articles. More particularly, the invention relates to a method and apparatus for transferring a plastic parison from an extrusion station to a blow molding station, using either the blow molds, preform molds, or both, to transfer the parison, without transferring the mechanism for applying the clamping forces to the molds.
There exist various methods for forming blown thermoplastic articles. In general, one of these methods includes injection molding a parison and then blowing the parison into the completed article. In other methods, an extruded parison is blown into the shape of the final article, either with or without an intermediate blowing step to form a preform, for purposes well known in the art.
These and other methods and devices employ a variety of mechanisms to transfer the molds and the parisons as well as to clamp the molds under high pressure for the blowing operation. Typically, the molds are mounted on tie rods in order to distribute the clamping forces. The use of four or more tie rods would be preferable; however, since it is necessary to move parts into and out of the clamp, such an arrangement is generally not feasible since the tie rods obstruct movement of parts into and out of the clamp area or between the extrusion and molding stations. Thus, clamps have been devised where only two or three tie rods are used, while others have tie rods that mechanically clamp together after a part has passed through. Still other machines use a C-clamp arrangement such that there are no top tie rods.
In most prior devices the blow molds are permanently fastened within the clamps, such that the heavy clamps travel with the molds. For example, in prior extrusion blow molding processes the molds and the heavy hydraulic mechanism for exerting force travel from station to station as the parison is transferred to different processing stations.
These and other devices and methods include certain inherent drawbacks. For example, those prior devices typically cannot employ a sufficient number of tie rods to equally distribute final clamping forces for the blowing operation. Prior devices and methods also require transfer of heavy clamping mechanisms, thus increasing production costs and reducing efficiency. Other devices and methods fail to provide alternate methods of support for the blanks, thus reducing flexibility in handling. Still other devices and methods encounter problems in positioning the blanks in the various stations. Yet others fail to properly control the forces applied in handling the blanks or partially formed parisons thus further reducing flexibility with regard to heat transfer and other process conditions.